Overview of GRE
Definition
If IPv4 GRE and IPv6 GRE implement a feature in the same way, details are not provided in this chapter. For details about implementation differences, see Appendixes.
Generic Routing Encapsulation (GRE) is a tunneling protocol that encapsulates the packets of a wide variety of network layer protocols, such as Internetwork Packet Exchange (IPX), IPv6, and AppleTalk, into IP tunneling packets. Then these packets can be transmitted over an IPv4 network.
GRE provides a mechanism of encapsulating packets of a protocol into packets of another protocol. This allows packets to be transmitted over heterogeneous networks. The channel for transmitting heterogeneous packets is called a tunnel.
GRE tunnel with the one-dimensional tunnel interface: also called distributed GRE tunnel. The tunnel interface is one-dimensional (named only by the interface number). GRE packets are encapsulated and decapsulated directly on the inbound interface board. If the multi-field classification and CAR services are configured simultaneously, bandwidth that services consume may double.
Purpose
To allow the packets of a wide variety of network layer protocols, such as IPX, IPv6, and AppleTalk, to be transmitted over the IPv4 network, GRE is introduced. GRE solves the transmission problem faced by heterogeneous networks.
In addition, GRE serves as a Layer 3 tunneling protocol of VPNs, and provides a tunnel for transparently transmitting VPN packets. Currently, GRE is supported by IPv4 L3VPN, but not IPv6 L3VPN.
Benefits
GRE has low requirements for device performance and allows devices that do not support Multiprotocol Label Switching (MPLS) to establish tunnels.